1. Field of the invention
The present invention relates to a developer for developing an electrostatic image, an image forming method, and a heat fixing method for fixing a toner image.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are hitherto known as method for carrying out electrophotography, which, in general, is a process in which copies are obtained by forming an electrostatic latent image on a photosensitive member by various means utilizing a photoconductive material, developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. Methods for development using toners or methods of fixing toner images have been hitherto proposed in variety, and methods suited for any respective image-forming processes have been employed. In recent years, on such electrophotography, there is a demand for higher-speed copying and higher image quality.
As methods of producing toners, it is commonly known to use a process comprising melt-kneading a thermoplastic resin, a colorant such as a dye or a pigment and additives such as a charge control agent to effect their uniform dispersion, thereafter cooling the melt-kneaded product, pulverizing the cooled product by means of a pulverizer, and classifying the pulverized product by means of a classifier to give a toner having the desired particle diameter.
In the toners produced through the step of such pulverization, there is a limit in faithfully reproducing the latent image since in general their particles lack definite form, i.e., are amorphous. In order to achieve a high image quality using the toners produced by such pulverization, it is necessary to pulverize particles in a smaller diameter. However, making particle diameter smaller makes it necessary to use more energy and tends to make poor the yield of toner.
In addition, in the toners produced by such pulverization, there are limitations when a release material (a material with release properties) such as wax is added. For example, in order to make the release material have a dispersibility on a satisfactory level, there are limitations such that i) the material is not dissolved into a liquid state in the range of the temperature at which it is kneaded together with the resin, and ii) the release material must be contained in an amount not more than a given amount. Because of such limitations, there is a limit in improving the fixing performance of the toners produced by pulverization.
To cope with the problems in such amorphous toners, spherical toners have been proposed. For example, Japanese Patent Publication No. 56-13945 discloses a method of obtaining a spherical toner by melt-spraying. Japanese Patent Publication No. 57-51676 discloses a method of obtaining a spherical toner by adding to an amorphous toner an organic solvent in a small quantity followed by stirring under cooling. Japanese Patent Publication No. 36-10231 and Japanese Patent Applications Laid-open No. 59-53856 and No. 59-61842 also disclose a method of obtaining a spherical toner by suspension polymerization.
These spherical toners have uniform particle shapes and hence can readily adhere faithfully to the latent image. In particular, no minute irregularity occurs at the edges of the latent image to give a high image quality. In the case when the spherical toner is obtained by suspension polymerization, the toner particles can be readily made to have a smaller particle diameter and can be more suitable for achievement of a higher image quality.
The toner obtained by suspension polymerization (hereinafter "polymerized toner"), when compared with amorphous toners obtained by pulverization, can readily have a function of a capsular structure and hence can encapsulate wax in a large quantity, so that a good fixing performance and anti-offset properties can be expected.
As for the spherical toners, they tend to cause a deterioration of their performance even if various additives are used, making it difficult to obtain toners with a satisfactory durability. They also so strongly adhere to a photosensitive member that the toner cleaning after the transfer step tends to become insufficient. Several reports are seen on such problems.
In the method using suspension polymerization, toner particles are formed by dispersing in a dispersion medium as typified by water a polymerizable monomer composition substantially incompatible therewith, followed by polymerization. In order to obtain a toner with a sharp particle size distribution, it is a very important subject how stably droplets of the polymerizable monomer composition having been suspended in this aqueous dispersion medium, i.e., polymerizable monomer composition particles, are kept constant in diameter in the course of the polymerization.
To settle this subject, it is very important to make researches on dispersion stabilizers capable of imparting an appropriate surface tension to the interfaces between the droplets of a polymerizable monomer composition and the dispersion medium without adversely affecting environmental properties of toners as exemplified by moisture resistance. It is also very important how to conduct a post-treatment.
In recent years, copying apparatus or printers are not only used as a copying machine for office work to merely take copies of originals, but also has begun to be used in the field of printers serving as outputs of computers and in the field of personal copying of private use.
Under such circumstances, the apparatus are severely sought to be made small-sized, lightweight and of low power consumption, and copying machines have now been formed of more simple components. For example, as methods of developing electrostatic latent images, there are the two-component development, which makes use of a mixture comprised of a toner and a carrier, and the one-component development, which makes use of only a toner.
Non-magnetic one-component development as disclosed in Japanese Patent Applications Laid-open No. 58-116559, No. 60-120368 and No. 63-2711371 have attracted notice as development methods that can solve the problems discussed above.
In such non-magnetic one-component development, a developer is coated on a developer carrying member by means of a blade or the like to form a coat layer. The developer is electrostatically charged as a result of its friction with the blade or the surface of the developer carrying member. If the developer is coated in a thick layer, part of the developer can not be sufficiently charged, which causes fogging or toner scatter, and hence the developer must be coated in a thin layer. For this reason, the blade must be brought into pressure contact with the developer carrying member at a sufficient pressure. The force the developer receives at this time is larger than the force a developer receives in the two-component development or the one-component development making use of a magnetic toner. Hence the developer tends to be deteriorated and image deterioration such as fogging or density decrease tends to occur.
The developer used in the non-magnetic one-component development is required to have a large mechanical strength and thermal strength. However, an attempt to merely increase these strengths results in an increase in the heat energy required for the fixing, which is contradictory to the demand for the low power consumption. Thus, in the non-magnetic one-component development, higher performances are sought in both developing performance and fixing performance.
As a method of fixing a visible toner image to a recording medium, a heat-roll fixing system is widely used, in which a recording medium holding thereon a visible toner image having not been fixed is heated while it is held and carried between a heat roller maintained at a given temperature and a pressure roller having an elastic layer and coming into pressure contact with the heat roller. A belt fixing system is also known, as disclosed in U.S. Pat. No. 3,578,797.
The heat-roll fixing, however, has the following disadvantages:
(1) A time during which an image-forming operation is prohibited, i.e., what is called a waiting time, is required until the heat roller reaches a given temperature. PA0 (2) The heat roller must be maintained at an optimum temperature in order to prevent poor fixing caused by the variations of the heat-roller temperature that may occur when the recording medium is passed or because of other external factors, and also to prevent the transfer of toner to the heat roller, i.e., what is called the offset phenomenon. This makes it necessary to make large the heat capacity of the heat roller or a heater element, which requires a large electric power and also causes in-machine temperature rise in the image forming apparatus. PA0 (3) After the recording medium has been passed over the heat roller, the recording medium and the toner on the recording medium are slowly cooled because of a high temperature of the heat roller, resulting in a state in which a high adhesion of the toner is maintained. Thus, conjointly with the curvature of the roller also, there may often occur offset, or paper jam caused by the winding of the recording medium around the roller. PA0 (4) A protective member must be provided on account of safety since there is a possibility of direct touch to the high-temperature heat roller.
The above problems (1) and (2) in the heat-roll fixing are not fundamentally solved also in the belt fixing system disclosed in U.S. Pat. No. 3,578,797.
Japanese Patent Application Laid-open No. 63-313182 discloses an image forming apparatus with a shorter waiting time and a low power consumption, comprising a fixing unit in which a visible toner image is heated via a movable heat-resistant sheet by means of a heating element having a low heat capacity, pulsewise generating heat by electrification, and is thus fixed to a recording medium. Japanese Patent Application Laid-open No. 1-187582 discloses a fixing unit for heat-fixing a visible toner image on a recording medium via a heat-resistant sheet, wherein said heat-resistant sheet comprises a heat-resistant layer and a release layer or a low-resistant layer, thereby effectively preventing the offset phenomenon.
In addition to the factors in the above fixing apparatus, however, achievement of both the excellent fixing performance of a visible toner image to a recording medium and the prevention of offset and simultaneous realization of a fixing method with a shorter waiting time and a low power consumption are greatly concerned with the properties of a toner. Thus, it is sought to provide a toner suited therefor.